The invention generally relates to wireless local area networks (WLAN) and to cellular networks. In particular, embodiments of the invention relate to roaming of a dual-mode mobile communication device from a WLAN to a cellular network.
In accordance with the Institute of Electrical and Electronics Engineers (IEEE) 802.11 standards, wireless communication between a mobile station (STA) and an access point (AP) is possible at data rates that are predefined in the standards. IEEE standard 802.11b permits communication in 1, 2, 5.5 and 11 Mbps (Mega bits per second). IEEE standard 802.11a permits communication in 6, 9, 12, 18, 24, 36, 48 and 54 Mbps. IEEE standard 802.11g permits communication in any of the data rates defined in IEEE standard 802.11b and IEEE standard 802.11a, and dictates that support for 1, 2, 5.5, 11, 6, 12 and 24 Mbps is mandatory.
The intent behind multiple rates is to permit rate usage which best suits the current STA/AP link quality. Various factors may affect the quality of the link, including, for example, the distance between the STA and the AP, and increases of electromagnetic noise over the medium connecting the STA and the AP. As the quality of the link decreases, mobile stations will drop to lower rates, which require less signal-to-noise ratio (SNR) for reliable packet transmission performance. The link rate selection algorithm is not specified in IEEE 802.11 but is left up to vendor implementations.
In IEEE 802.11 the STA is responsible for triggering handoffs. A negative consequence of this is that a STA may continue to use a link and operate at data rates which are lower than what may be available should the STA handoff to a different AP. That is, in response to decreases in link quality, the station may continue to drop its data rate rather than to scan for an AP which could provide a higher rate link. When this is true the capacity of the network is affected since the station is continuing to use an unnecessarily low data rate.
To prevent capacity loss due to these effects, some APs prevent stations from communicating with them at data rates below certain specified site-dependent values, even though this contradicts the requirements of current IEEE 802.11 standards. STAs are forced to search for a better AP when these limits are reached. For example, in IEEE 802.11, an AP advertises supported data rates in its Beacon broadcasts. These rates are part of its “BSS Basic Rate Set”. By specifying a minimum supported rate the AP can enforce a lower limit on the data rates used in its basic service set (BSS). This will help prevent the capacity-lowering behaviour discussed above. The minimum rate specified by the AP would typically be a function of the surrounding Wi-Fi coverage deployment, i.e., in a dense deployment where low quality links are rare, the minimum advertised data rate could be set higher than in a sparse deployment. The above mechanism is geared towards maintaining high capacity in heavy coverage situations.
However, when leaving the area covered by APs of a WLAN, a STA may not be able to find another WLAN to connect to. If the STA is capable also of cellular communication, it may search for a cellular base station and may connect to one if found. A handover from a WLAN to a cellular network may take quite some time, and the mobile station may lose the connection with the WLAN before establishing a connection with the cellular network. If the mobile station is involved in a real-time communication application, for example, a telephone conversation, the application may be interrupted.
It will be appreciated that for simplicity and clarity of illustration, elements shown in the figures have not necessarily been drawn to scale. For example, the dimensions of some of the elements may be exaggerated relative to other elements for clarity.